(1) Technical Field
Various embodiments described herein relate to receivers and more particularly to Radio Frequency Front End (RFFE) with low noise amplifiers for use in communications equipment configured for receiving carrier aggregation signals.
(2) Background
Many modern electronic systems include radio frequency (RF) transceivers capable of transmitting and receiving signals; examples include personal computers, wireless tablets, cellular telephones, wireless network components, televisions, cable system “set top” boxes, radar systems, etc. In communication systems that rely upon such transceivers, radio frequencies are separated into frequency bands assigned to a particular frequency range. For example, the IEEE (Institute of Electrical and Electronics Engineers) defines the following bands:
HF0.003 TO 0.03 GHzHIGH FREQUENCYVHF0.03 TO 0.3 GHzVERY HIGH FREQUENCYUHF0.3 TO 1 GHzULTRA HIGH FREQUENCYL1 TO 2 GHzLONG WAVES2 TO 4 GHzSHORT WAVEC4 TO 8 GHzCOMPROMISE BETWEEN S AND XX8 TO 12 GHzUSED IN WW II FOR FIRE CONTROL, X FOR CROSS (AS IN CROSSHAIR). EXOTIC.Ku12 TO 18 GHzKURZ-UNDERK18 TO 27 GHzKURZ (GERMAN FOR “SHORT”)KA27 TO 40 GHzKURZ-ABOVEV40 TO 75 GHzW75 TO 110 GHzW FOLLOWS V IN THE ALPHABETMM 110 TO 300 GHzMILLIMETEROR G
One example of a modern electronic system that relies upon transceivers that transmit and receive RF signals is the cellular telephone system. For maximum compatibility in North American 2G/3G/4G, cellular telephones are typically capable of handling dual-band 800 MHz Cellular or 1900 MHz PCS signals. In many markets, 4G data (LTE, WiMAX) transmitted and received by such cellular telephones is modulated on signals operating at frequencies of 700 MHz, 1700-2100 MHz, 1900 MHz and 2500-2700 MHz. Channels are assigned to a narrower range of frequencies within each band. Typically, RF signals to be transmitted are modulated within one of the channels of a selected band.
Radio frequency (RF) transceivers capable of receiving such signals comprise a receiver front end circuit that typically includes a low noise amplifier (“LNA”). The LNA is responsible for providing the first stage amplification to a signal received within the communications receiver. The operational specifications of the LNA are very important to the overall quality of the communications receiver. Any noise or distortion in the input to the LNA will get amplified and cause degradation of the overall receiver performance. Accordingly, the sensitivity of a receiver is, in large part, determined by the quality of the receiver front end circuit and in particular, by the quality of the LNA.
In some cases, such as the case of cellular telephones noted above, the LNA is required to operate over a relatively broad frequency band and to amplify signals having several modulated baseband or intermediate frequency (IF) signals. In some cases, the LNA of a cellular telephone may be required to amplify a received signal having multiple modulated IF or baseband signals. For example, some cellular telephones are required to receive an intraband noncontiguous carrier aggregation (CA) signal. A CA signal can have two channels (or IF carriers) having frequencies that are not adjacent to one another, but which lie in the same frequency band. For example, a CA signal may have two non-adjacent channels within a cellular frequency band defined by 3rd Generation Partnership Project (3GPP), a well-known industry standard setting organization.
In the case in which a receiver is required to receive a CA signal, such as a cellular telephone that is compliant with the Release 11 of the 3GPP communications industry standard, the LNA typically amplifies the received signal and provides the amplified output signal to a passive splitter.
FIG. 1 is an illustration of a portion of a cellular telephone receiver front end circuit in which an LNA 101 is coupled to a variable attenuator 103. A bypass switch 105 allows the variable attenuator to be optionally shunted. The signal is then coupled to a single pole, three throw mode selector switch 107 that allows the output of the LNA 101 to be selectively coupled to only a first downconverter and baseband circuitry (DBC) 109, a second DBC 111 or both the first and the second DBC 109, 111.
When the mode selector switch 107 is in the first position (i.e., Single Channel mode 1), the output of the LNA 101 is coupled directly to the first DBC 109. In the second position (i.e., Split mode), the output of the LNA 101 is coupled through a passive power splitter 113 to both the first and second DBC 109, 111. In the third position (i.e., Single Channel mode 2), the output of the LNA 101 is coupled to only the second DBC 111.
Several limitations arise from the architecture shown in FIG. 1. The first limitation is the amount of isolation that can be achieved between the first and second DBC 109, 111. Typically, a well-manufactured 3 dB splitter can achieve approximately 18-20 dB of isolation between outputs at the center frequency for which the splitter 113 is designed to operate. Signals that are cross-coupled from one DBC to the other will typically result in interference and distortion that will result in an overall reduction in sensitivity of the receiver.
Furthermore, passive splitters typically are designed to operate optimally in a relatively narrow frequency range. That is, passive splitters, by their nature are narrow band devices. As the frequency of the signal coupled through the splitter 113 deviates from the optimal frequency for which the splitter was designed, the output-to-output isolation will degrade. Due to the limitations of the splitters currently available, and because receivers that are designed to handle CA signals must operate in a relatively broad frequency range, the desired isolation between the DBCs 109, 111 is difficult to achieve.
Furthermore, power splitters such as the splitter 113 shown in FIG. 1, have significant loss. Since 3 dB power splitters split the power in half, even an ideal splitter will result in a 3 dB reduction in power. In addition, most splitters will have an additional 1.0 to 1.5 dB of insertion loss. The insertion loss, like the output-to-output isolation, will typically get worse as the frequency of the signals applied deviates from the center frequency for which the splitter was designed to operate.
Still further, the losses encountered in the mode selection switch 107 and the splitter 113 lead to a need for more gain. This results in reductions in linearity (as typically characterized by measuring the “third order intercept”) and degradation of the noise figure of the receiver when operating in Split mode.
Therefore, there is a currently a need for a CA capable receiver front end circuit that can operate in Split mode with high output-to-output isolation, without degraded third order intercept and noise figure, and with relatively low front end losses.
Still further, in several cases today, it is necessary to have more than two inputs, each of which may receive intraband (Intra-B) CA signals or an interband (Inter-B) CA signals at different frequencies. Due to limitations in the capability of the LNAs to handle broad frequency ranges, it may be necessary to have several LNAs, each tuned to amplify signals in a particular frequency range. However, restrictions on the size of a receiver front end circuit in which the LNA resides may place limits on the number of LNAs that can be present, or at least make it necessary to efficiently use the real estate in the integrated circuits of the FEC.
Accordingly, there is currently a need for an efficient, flexible FEC capable of handling several possible signals, including Intra-B CA signals in different frequency ranges and Inter-B CA signals in different frequency ranges, as well as non-CA signals.